1. Field of the Invention
The invention relates to electrical conductors. More particularly, the invention relates to electrical conductors which exhibit low resistance, spatial efficiency, low weight, good flexibility, enhanced bandwidth, minimized parasitic capacitance and inductance, and which are well suited for use, for example, in VHF and UHF transmission lines as well as in coils, solenoids, motors, and transformers.
2. State of the Art
Parent application Ser. No. 08/843,405 which is referenced above describes the general techniques known in the art for making electrical cables from helically twisted filaments, and proposes methods of twisting and drawing wire cables for enhancing the conductivity, flexibility and tensile strength of the cables. In addition to low resistance, flexibility and tensile strength, other characteristics of cables may be important depending on the application in which the cable is used. For example, the ability of a cable to remain cool during operation is often an important consideration. For cables used outdoors for power transmission, renitence to corrosion and low weight of the cable are important considerations. For cables which are subjected to repeated flexion, good flexibility as well as high fatigue strength are important. In cables which are used as leads for semiconductors and other electronic components, parasitic capacitance and inductance are important considerations.
Parent application Ser. No. 08/963,686 which is referenced above discloses cables made from plated filaments which are first twisted together and then drawn through reducing dies (or swaged), filaments which are twisted together around a core material which melts or deforms during drawing of the cable through reducing dies, filaments which are twisted around a tube prior to drawing through reducing dies, and cables which are made from combinations of these methods. The cables exhibit a conductivity comparable to cables having greater diameter and weight. The smaller diameter of the cables of the invention allows them to be used as leads for electronic components in order to achieve reduced parasitic capacitance without increased resistivity or reactance or component package size. The cold working of the cables of the invention provides them with enhanced flexibility and fatigue strength. The combination of materials used in the cables of the invention provides them with renitence to corrosion and the adverse affects of aging as well as enhanced conductivity. Cables formed with a hollow tube core can be self-cooling, or easily cooled by flowing a coolant through the hollow core. The hollow tube core also enhances fatigue strength, resists the effects of aging, and lowers the weight of the cable. Cables formed with a silver core are also self-cooling.
Both of the parent applications recognize that multi-stranded electrical cable is generally more flexible than a single strand conductor which has similar conductive capacity. It is also recognized in the parent applications that multi-stranded cables have several disadvantages compared to single strand conductors. In particular, the parent applications teach that multi-stranded cables are spatially inefficient and possess self-induced parasitic inductance because of the helical paths of the strands which are not in perfect contact with each other. It is also recognized that the helical paths of the strands results in a longer conductive path (known as the "lay effect") and a corresponding increase in resistivity.
The physical properties of multi-stranded electrical cable also cause poor performance at very high frequencies (VHF) and ultra high frequencies (UHF). Signal losses at these frequencies are the result of multiple signal reflections along the length of a multi-stranded transmission line. Reflections occur where the cable exhibits an abrupt change in impedance due to the imperfect contact of the strands with each other. The reflected signals are typically out of phase with the transmitted signal and interfere destructively with the transmitted signal. This results in a "smearing" of signal pulses which limits the bandwidth of the transmission line.
The twisted and drawn multi-stranded wires of the parent applications maximize the spatial efficiency of a generally cylindrical conductor and achieve many other advantages as described above. However, there are certain applications where a generally cylindrical conductor is not the most spatially efficient. For example, where a conductor is wound to form a coil, a cylindrical cross-section is not necessarily the most spatially efficient.
The twisted and drawn multi-stranded wires of the parent applications also generally possess enhanced flexibility. However, certain electrical coils require relatively large diameter conductors wound to a relatively small radius. Winding a large diameter conductor to form a small diameter coil is difficult because the large diameter conductor may not have the flexibility to be wound so tightly. Use of a multi-strand conductor for such a coil introduces other problems regarding conductivity as described above. Moreover, it is usually desirable that the finished coil be inflexible. In addition, when a multi-strand conductor is formed into a non-circular cross section, individual strands are pinched irregularly such that their cross sections change along their length. This change in cross sectional shape (even if cross sectional area remains the same) increases the resistivity of the conductor (known as the "pinch effect").